The present invention relates to a bimetal driven control valve in which bimetal deformation controls a flow rate.
Bimetal driven control valves are described in U.S. Pat. Nos. 3,397,552, 3,478,534 and 3,967,781. In the control valves described in the above patents, the bimetals are deformed by changes in ambient temperature so that precise control in places where changes in the ambient temperature are great cannot be performed.
Japanese Laid-Open patent application No. 55-6025 describes a bimetal driven control valve which eliminates the influence of changes in the ambient temperature. FIG. 1 of this application is a vertical cross-sectional view of a bimetal driven valve device and FIG. 2 is a plan view of the bimetal plate used in the device of FIG. 1, to which reference is herein made. In this control valve which is shown in vertical cross-section in FIG. 9 of the accompanying drawings, respective ends of a pair of bimetals 2 and 3 are mounted on the inner surface of a housing 1. The pair of bimetals 2 and 3 are disposed substantially parallel to each other. Respective metals ( ), of low thermal expansion coefficient, oppose each other. The pair of bimetals 2 and 3 are interconnected by a rigid body 5 to which both ends of the bimetals are connected. When the ambient temperature rises, one bimetal 2 tends to bend upward in a convex shape, while the other bimetal 3 tends to bend downward in a convex shape. As a result, according to the inventor of this application, the stress generated at the bimetal 2 is balanced by the stress generated at the bimetal 3 so that the position of a valve rod 17 does not change.
In practice, however, when the ambient temperature rises, the position of the valve rod 17 changes.
This is because of the fact that when the ambient temperature rises, the stress distributions respectively generated at the bimetals 2 and 3 prevent the formation of a smooth arc extending upward in a convex shape at the bimetal 2 and a smooth arc extending downward in a convex shape at the bimetal 3. The pair of bimetals 2 and 3 are respectively deformed in a sinusoidal shape due to the stress, since the bimetals 2 and 3 are interconnected by the housing 1 and the rigid body 5. When the pair of bimetals 2 and 3 deform in this way, the position of the valve rod 17 changes. In this embodiment, obviously the bimetal 2 cannot be bent to form a smooth arc extending upward even when it is heated by the heating means 4. Thus, a precise control of flow rate cannot be achieved by the vertical movement of the valve rod 17.
In this control valve, the respective ends of the pair of bimetals 2 and 3 are mounted on the inner surface of the housing 1, and the pair of bimetals 2 and 3 must be interconnected by the rigid body 5. With this construction, the assembly process of the control valve is complicated, and the manufacturing cost of the control valve increased.
In FIG. 3 of Japanese Laid-Open patent application No. 56-3859 which is a vertical cross-sectional view of a pilot valve and to which reference is herein made, an improvement in the control valve of Japanese Laid-Open patent application No. 55-6,025 described above is described. In this improved control valve which is shown in vertical cross-section in FIG. 10 of the accompanying drawings, the respective ends of a pair of bimetals 31 and 32 are not mounted to the inner surface of a housing 30. The pair of bimetals 31 and 32 are interconnected by a pair of rigid bodies 33 and 34. In the pair of bimetals 31 and 32 which are spaced apart and substantially parallel to each other, a metal ( ) of high thermal expansion coefficient of the bimetal 31 opposes a metal (.quadrature.) of low thermal expansion coefficient of the bimetal 32. When the ambient temperature rises, the bimetal 31 tends to bend downward in a convex shape, while the bimetal 32 tends to bend downward in a convex shape. The displacement of the bimetal 31 is equal to that of the bimetal 32, so the position of a valve body 36 does not change.
In order to balance the displacement of the bimetal 31 with that of the bimetal 32, the pair of bimetals 31 and 32 must be disposed substantially parallel to each other. However, with this construction, in a unit constituted by the pair of bimetals 31 and 32 which are held substantially parallel to each other by a pair of the rigid bodies 33 and 34, the parallel arrangement of the pair of bimetals 31 and 32 may be destroyed when an external force is imposed on the control valve before or during assembly. Therefore, after assembly of the control valve, an operation for reforming the parallel arrangement of the bimetals is often required. This reforming operation complicates the assembly process of the control valve. Due to the external force imposed on the bimetals, the pair of bimetals 31 and 32 which are disposed substantially parallel to each other may show metal fatigue at a considerably earlier period of time. The parallel arrangement of the pair of bimetals 31 and 32 is disturbed in a comparatively earlier period of application of the control valve. As a result, in a comparatively shorter period of time, changes in the flow rate in accordance with changes in the external temperature tend to occur and precise flow rate control is not achieved.